Chapter 12 Exception Handling and Text IO

Motivations When a program runs into a runtime error, the program terminates abnormally. How can you handle the runtime error so that the program can continue to run or terminate gracefully? This is the subject we will introduce in this chapter.

Objectives To get an overview of exceptions and exception handling (§12.2). To explore the advantages of using exception handling (§12.2). To distinguish exception types: Error (fatal) vs. Exception (nonfatal) and checked vs. unchecked (§12.3). To declare exceptions in a method header (§12.4.1). To throw exceptions in a method (§12.4.2). To write a try-catch block to handle exceptions (§12.4.3). To explain how an exception is propagated (§12.4.3). To obtain information from an exception object (§12.4.4). To develop applications with exception handling (§12.4.5). To use the finally clause in a try-catch block (§12.5). To use exceptions only for unexpected errors (§12.6). To rethrow exceptions in a catch block (§12.7). To create chained exceptions (§12.8). To define custom exception classes (§12.9). To discover file/directory properties, to delete and rename files/directories, and to create directories using the File class (§12.10). To write data to a file using the PrintWriter class (§12.11.1). To use try-with-resources to ensure that the resources are closed automatically (§12.11.2). To read data from a file using the Scanner class (§12.11.3). To understand how data is read using a Scanner (§12.11.4). To develop a program that replaces text in a file (§12.11.5). To read data from the Web (§12.12). To develop a Web crawler (§12.13).

Exception Handling The exception handling in java is one of the powerful mechanism to handle the runtime errors so that normal flow of the application can be maintained. An exception is an event that occurs during the execution of a program that disrupts the normal flow of instructions. Run time error occurs during the execution of a program. In contrast, compile-time errors occur while a program is being compiled. Runtime errors indicate bugs in the program or problems that the designers had anticipated but could do nothing about. For example, running out of memory will often cause a runtime error.

Difference between error & exception Errors indicate serious problems and abnormal conditions that most applications should not try to handle. Error defines problems that are not expected to be caught under normal circumstances by program. For example memory error, hardware error, JVM error etc. Exceptions are conditions within the code. A developer can handle such conditions and take necessary corrective actions. Few examples – DivideByZero exception, NullPointerException, ArithmeticException, ArrayIndexOutOfBoundsException

Try Catch in Java – Exception handling A try block is always followed by a catch block, which handles the exception that occurs in associated try block/ A catch block must be associated with a try block

class Nest{ public static void main(String args[]){ //Parent try block try{ //Child try block1 try{ //try-catch block inside another try block System.out.println("Inside block1"); int b =45/0; System.out.println(b); } catch(ArithmeticException e1){ //Exception Message System.out.println("Exception: e1"); } //Child try block2 try{ //try-catch block inside another try block System.out.println("Inside block2"); int b =45/0; System.out.println(b); } catch(ArrayIndexOutOfBoundsException e2){ //Exception Message System.out.println("Exception: e2"); } System.out.println("Just other statement"); } catch(ArithmeticException e3){ //Catch of Main(parent) try block //Exception Message System.out.println("Arithmetic Exception"); System.out.println("Inside parent try catch block"); } catch(ArrayIndexOutOfBoundsException e4){ System.out.println("ArrayIndexOutOfBoundsException"); System.out.println("Inside parent try catch block"); } catch(Exception e5){ System.out.println("Exception"); System.out.println("Inside parent try catch block"); } System.out.println("Next statement.."); } } Flow of try catch block program can also contain nested try-catch-finally blocks.

Java Document by Oracle Weblink catch, try, throw The Catch or Specify Requirement How to catch and try handle exceptions. Also, the throw statement and the Throwable class and its subclasses. The try and catch Block - Weblink Java Document by Oracle Weblink try and catch question and answer -- Weblink

Exception-Handling Overview Show runtime error Fix it using an if statement Quotient QuotientWithIf import java.util.Scanner; public class Quotient { public static void main(String[] args) { Scanner input = new Scanner(System.in); // Prompt the user to enter two integers System.out.print("Enter two integers: "); int number1 = input.nextInt(); int number2 = input.nextInt(); System.out.println(number1 + " / " + number2 + " is " + (number1 / number2)); } } import java.util.Scanner; public class QuotientWithIf { public static void main(String[] args) { Scanner input = new Scanner(System.in); // Prompt the user to enter two integers System.out.print("Enter two integers: "); int number1 = input.nextInt(); int number2 = input.nextInt(); if (number2 != 0) System.out.println(number1 + " / " + number2 + " is " + (number1 / number2)); else System.out.println("Divisor cannot be zero "); } } With a method QuotientWithMethod import java.util.Scanner; public class QuotientWithMethod { public static int quotient(int number1, int number2) { if (number2 == 0) { System.out.println("Divisor cannot be zero"); System.exit(1); } return number1 / number2; }

Exception Types

System Errors System errors are thrown by JVM and represented in the Error class. The Error class describes internal system errors. Such errors rarely occur. If one does, there is little you can do beyond notifying the user and trying to terminate the program gracefully.

Exceptions Exception describes errors caused by your program and external circumstances. These errors can be caught and handled by your program.

Runtime Exceptions RuntimeException is caused by programming errors, such as bad casting, accessing an out-of-bounds array, and numeric errors.

Checked Exceptions vs. Unchecked Exceptions RuntimeException, Error and their subclasses are known as unchecked exceptions. All other exceptions are known as checked exceptions, meaning that the compiler forces the programmer to check and deal with the exceptions.

Unchecked Exceptions Unchecked exception.

Declaring, Throwing, and Catching Exceptions

Declaring Exceptions Every method must state the types of checked exceptions it might throw. This is known as declaring exceptions. public void myMethod() throws IOException throws IOException, OtherException

Throwing Exceptions When the program detects an error, the program can create an instance of an appropriate exception type and throw it. This is known as throwing an exception. Here is an example, throw new TheException(); TheException ex = new TheException(); throw ex;

Throwing Exceptions Example /** Set a new radius */ public void setRadius(double newRadius) throws IllegalArgumentException { if (newRadius >= 0) radius = newRadius; else throw new IllegalArgumentException( "Radius cannot be negative"); }

Catching Exceptions try { statements; // Statements that may throw exceptions } catch (Exception1 exVar1) { handler for exception1; catch (Exception2 exVar2) { handler for exception2; ... catch (ExceptionN exVar3) { handler for exceptionN;

Catching Exceptions

Catch or Declare Checked Exceptions Suppose p2 is defined as follows:

Catch or Declare Checked Exceptions Java forces you to deal with checked exceptions. If a method declares a checked exception (i.e., an exception other than Error or RuntimeException), you must invoke it in a try-catch block or declare to throw the exception in the calling method. For example, suppose that method p1 invokes method p2 and p2 may throw a checked exception (e.g., IOException), you have to write the code as shown in (a) or (b).

Example: Declaring, Throwing, and Catching Exceptions Objective: This example demonstrates declaring, throwing, and catching exceptions by modifying the setRadius method in the Circle class defined in Chapter 8. The new setRadius method throws an exception if radius is negative. TestCircleWithException CircleWithException Run

TestCircleWithException public class CircleWithException { /** The radius of the circle */ private double radius; /** The number of the objects created */ private static int numberOfObjects = 0; /** Construct a circle with radius 1 */ public CircleWithException() { this(1.0); } /** Construct a circle with a specified radius */ public CircleWithException(double newRadius) { setRadius(newRadius); numberOfObjects++; } /** Return radius */ public double getRadius() { return radius; } /** Set a new radius */ public void setRadius(double newRadius) throws IllegalArgumentException { if (newRadius >= 0) radius = newRadius; else throw new IllegalArgumentException( "Radius cannot be negative"); } /** Return numberOfObjects */ public static int getNumberOfObjects() { return numberOfObjects; } /** Return the area of this circle */ public double findArea() { return radius * radius * 3.14159; } } public class TestCircleWithException { public static void main(String[] args) { try { CircleWithException c1 = new CircleWithException(5); CircleWithException c2 = new CircleWithException(-5); CircleWithException c3 = new CircleWithException(0); } catch (IllegalArgumentException ex) { System.out.println(ex); } System.out.println("Number of objects created: " + CircleWithException.getNumberOfObjects()); } }

Rethrowing Exceptions try { statements; } catch(TheException ex) { perform operations before exits; throw ex;

The finally Clause try { statements; } catch(TheException ex) { handling ex; finally { finalStatements;

Trace a Program Execution animation Trace a Program Execution Suppose no exceptions in the statements try { statements; } catch(TheException ex) { handling ex; finally { finalStatements; Next statement;

Trace a Program Execution animation Trace a Program Execution The final block is always executed try { statements; } catch(TheException ex) { handling ex; finally { finalStatements; Next statement;

Trace a Program Execution animation Trace a Program Execution Next statement in the method is executed try { statements; } catch(TheException ex) { handling ex; finally { finalStatements; Next statement;

Trace a Program Execution animation Trace a Program Execution Suppose an exception of type Exception1 is thrown in statement2 try { statement1; statement2; statement3; } catch(Exception1 ex) { handling ex; finally { finalStatements; Next statement;

Trace a Program Execution animation Trace a Program Execution The exception is handled. try { statement1; statement2; statement3; } catch(Exception1 ex) { handling ex; finally { finalStatements; Next statement;

Trace a Program Execution animation Trace a Program Execution The final block is always executed. try { statement1; statement2; statement3; } catch(Exception1 ex) { handling ex; finally { finalStatements; Next statement;

Trace a Program Execution animation Trace a Program Execution The next statement in the method is now executed. try { statement1; statement2; statement3; } catch(Exception1 ex) { handling ex; finally { finalStatements; Next statement;

Trace a Program Execution animation Trace a Program Execution try { statement1; statement2; statement3; } catch(Exception1 ex) { handling ex; catch(Exception2 ex) { throw ex; finally { finalStatements; Next statement; statement2 throws an exception of type Exception2.

Trace a Program Execution animation Trace a Program Execution try { statement1; statement2; statement3; } catch(Exception1 ex) { handling ex; catch(Exception2 ex) { throw ex; finally { finalStatements; Next statement; Handling exception

Trace a Program Execution animation Trace a Program Execution try { statement1; statement2; statement3; } catch(Exception1 ex) { handling ex; catch(Exception2 ex) { throw ex; finally { finalStatements; Next statement; Execute the final block

Trace a Program Execution animation Trace a Program Execution try { statement1; statement2; statement3; } catch(Exception1 ex) { handling ex; catch(Exception2 ex) { throw ex; finally { finalStatements; Next statement; Rethrow the exception and control is transferred to the caller

When to Throw Exceptions An exception occurs in a method. If you want the exception to be processed by its caller, Then you should create an exception object and throw it. If you can handle the exception in the method where it occurs, there is no need to throw it.

When to Use Exceptions When should you use the try-catch block in the code? You should use it to deal with unexpected error conditions. Do not use it to deal with simple, expected situations. For example, the following code try { System.out.println(refVar.toString()); } catch (NullPointerException ex) { System.out.println("refVar is null");

When to Use Exceptions is better to be replaced by if (refVar != null) System.out.println(refVar.toString()); else System.out.println("refVar is null");

Custom Exception Class Example In Listing 13.8, the setRadius method throws an exception if the radius is negative. Suppose you wish to pass the radius to the handler, you have to create a custom exception class. InvalidRadiusException CircleWithRadiusException Run TestCircleWithRadiusException

The File Class The File class is intended to provide an abstraction that deals with most of the machine-dependent complexities of files and path names in a machine-independent fashion. The filename is a string. The File class is a wrapper class for the file name and its directory path.

Obtaining file properties and manipulating file

Problem: Explore File Properties public class TestFileClass { public static void main(String[] args) { java.io.File file = new java.io.File("image/us.gif"); System.out.println("Does it exist? " + file.exists()); System.out.println("The file has " + file.length() + " bytes"); System.out.println("Can it be read? " + file.canRead()); System.out.println("Can it be written? " + file.canWrite()); System.out.println("Is it a directory? " + file.isDirectory()); System.out.println("Is it a file? " + file.isFile()); System.out.println("Is it absolute? " + file.isAbsolute()); System.out.println("Is it hidden? " + file.isHidden()); System.out.println("Absolute path is " + file.getAbsolutePath()); System.out.println("Last modified on " + new java.util.Date(file.lastModified())); } } Problem: Explore File Properties Objective: Write a program that demonstrates how to create files in a platform-independent way and use the methods in the File class to obtain their properties. The following figures show a sample run of the program on Windows and on Unix. TestFileClass Run

Text I/O A File object encapsulates the properties of a file or a path, but does not contain the methods for reading/writing data from/to a file. In order to perform I/O, you need to create objects using appropriate Java I/O classes. The objects contain the methods for reading/writing data from/to a file. This section introduces how to read/write strings and numeric values from/to a text file using the Scanner and PrintWriter classes.

Writing Data Using PrintWriter WriteData Run

public class WriteData { public static void main(String[] args) throws Exception { java.io.File file = new java.io.File("scores.txt"); if (file.exists()) { System.out.println("File already exists"); System.exit(0); } // Create a file java.io.PrintWriter output = new java.io.PrintWriter(file); /* you can create PrintWriter objects for writing text to any file using print, println, and printf **/ // Write formatted output to the file output.print("John T Smith "); output.println(90); output.print("Eric K Jones "); output.println(85); // Close the file output.close(); } } WriteData

WriteDataWithAutoClose Try-with-resources Programmers often forget to close the file. JDK 7 provides the followings new try-with-resources syntax that automatically closes the files. try (declare and create resources) { Use the resource to process the file; } WriteDataWithAutoClose Run

Reading Data Using Scanner ReadData Run

Problem: Replacing Text Write a class named ReplaceText that replaces a string in a text file with a new string. The filename and strings are passed as command-line arguments as follows: java ReplaceText sourceFile targetFile oldString newString For example, invoking java ReplaceText FormatString.java t.txt StringBuilder StringBuffer replaces all the occurrences of StringBuilder by StringBuffer in FormatString.java and saves the new file in t.txt. ReplaceText Run

Reading Data from the Web Just like you can read data from a file on your computer, you can read data from a file on the Web.

Reading Data from the Web URL url = new URL("www.google.com/index.html"); After a URL object is created, you can use the openStream() method defined in the URL class to open an input stream and use this stream to create a Scanner object as follows: Scanner input = new Scanner(url.openStream()); ReadFileFromURL Run

Case Study: Web Crawler This case study develops a program that travels the Web by following hyperlinks.

Case Study: Web Crawler Add the starting URL to a list named listOfPendingURLs; while listOfPendingURLs is not empty { Remove a URL from listOfPendingURLs; if this URL is not in listOfTraversedURLs { Add it to listOfTraversedURLs; Display this URL; Exit the while loop when the size of S is equal to 100. Read the page from this URL and for each URL contained in the page { Add it to listOfPendingURLs if it is not is listOfTraversedURLs; } WebCrawler Run